Start by peeling the potatoes. Then slice them thin using a mandolin.
Put the potatoes into a container large enough to fit 4 quarts of water. Add the water. Place in the fridge overnight or for 8 hours.
Drain the potatoes out. Give them a rinse and then spin them dry in a salad spinner.
Place the potatoes in a large stock pot that is able to fit a gallon of milk.
Bring to a simmer over medium heat. Be careful you don't boil the milk as it will boil over and made a huge mess. Monitor closely.
Cook for 25 to 35 minutes at simmer or until the potatoes are fork tender.
Set aside 1 cup of the milk. Drain the potatoes, saving the rest of the milk if you want to use it for something else, otherwise discarding it.
Run the potatoes through a ricer and into a mixing bowl.
Add the butter and salt to taste. Then add in some of the reserved milk until it looks like it's enough. I didn't use it all. Use a hand or stand mixer to smooth out the potatoes, just for 15 to 30 seconds. Don't overwhip them. Taste and add more salt if you think they need it.